Intrinsic low friction polyoxymethylene

ABSTRACT

A tribologically modified polyoxymethylene polymer composition is disclosed. The polyoxymethylene polymer composition is comprised of a polyoxymethylene polymer and at least one tribological modifier. The tribological modifier may comprise at least one tribological modifier comprising an ultra-high molecular weight silicone having a kinematic viscosity of greater than 100,000 mm2 s−1. The composition may exhibit a dynamic coefficient of friction against a counter-material of from about 0.01 to about 0.15. The polyoxymethylene polymer compositions provide polymer articles with improved tribological properties and mechanical properties.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/511,891 filed on Oct. 10, 2014 which is based on and claims priorityto U.S. Provisional Application Ser. No. 61/893,474 having a filing dateof Oct. 21, 2013, which are both incorporated herein by reference intheir entirety.

BACKGROUND

Polyacetal polymers, which are commonly referred to as polyoxymethylenepolymers, have become established as exceptionally useful engineeringmaterials in a variety of applications. For instance, becausepolyoxymethylene polymers have excellent mechanical properties, fatigueresistance, abrasion resistance, chemical resistance, and moldability,they are widely used in constructing polymer articles, such as articlesfor use in the automotive industry and the electrical industry.

The mechanical properties of polyoxymethylene molding compositions areone of the reasons for their use in numerous applications. To improvetheir properties, polyoxymethylene polymers are often provided withadditives to adapt the properties for a specific application, forexample by using reinforcing fibers or tribological modifiers. Forinstance, polyoxymethylene polymers have been combined with atribological modifier for producing polymer compositions well suited foruse in tribological applications where the polymer article is in movingcontact with other articles, such as metal articles, plastic articles,and the like. These tribological applications can include embodimentswhere the polymer composition is formed into gear wheels, pulleys,sliding elements, and the like. The addition of a tribological modifiercan provide a composition with a reduced coefficient of friction, littlefrictional noise, and low wear.

In the past, high molecular weight polyolefins have been used to improvethe wear resistance of polyoxymethylene resins. For instance, U.S. Pat.No. 5,482,987, which is incorporated herein by reference in itsentirety, discloses a self-lubricating, low wear composition containinga polyoxymethylene and a lubricating system comprising a high molecularweight polyethylene, a high density polyethylene, and other components.U.S. Pat. No. 5,641,824, which is incorporated herein by reference inits entirety, discloses a self-lubricating melt blend of apolyoxymethylene and an ultra-high molecular weight polyethylene.However, polyoxymethylene compositions modified with these highmolecular weight polyethylene polymers may have a less than desirablesurface appearance as well as defects that may detract from the wearproperties of the compositions and articles produced therefrom.

Although polyoxymethylene polymers have been tribologically modified inthe past, further improvements are still necessary. For instance, a needexists for providing a polyoxymethylene polymer composition and apolymer article produced therefrom with improved tribologicalproperties. In particular, a need exists for providing apolyoxymethylene polymer composition and a polymer article producedtherefrom with a reduced coefficient of friction when in contact withother moving articles and improved wear properties.

SUMMARY

According to one embodiment, the present disclosure is directed to apolyoxymethylene polymer composition. The composition is comprised of apolyoxymethylene copolymer and at least one tribological modifiercomprising an ultra-high molecular weight silicone having a kinematicviscosity of greater than 100,000 mm² s⁻¹, wherein the polymercomposition is substantially free of silicone oil and wherein thepolymer composition has a dynamic coefficient of friction against acounter-material of from about 0.01 to about 0.15.

According to another embodiment, the present disclosure is directed to apolymer article. The polymer article is comprised of a polyoxymethylenepolymer composition comprising a polyoxymethylene polymer and at leastone tribological modifier comprising an ultra-high molecular weightsilicone having a kinematic viscosity of greater than 100,000 mm² s⁻¹,wherein the polymer composition is substantially free of silicone oiland wherein the polymer article exhibits a dynamic coefficient offriction against a counter-material of from about 0.01 to less than0.09.

According to one embodiment, the improved tribological properties of thepresent invention are exhibited between the composition or polymerarticle of the present invention and various counter-materials. Forinstance, the improved tribological properties and coefficient offriction may be exhibited between the composition or polymer article anda polyester surface such as a polyethylene terephthalate surface. Inanother embodiment, the improved tribological properties and coefficientof friction may be exhibited between the composition or polymer articleand a polyacetal surface, a metal surface such as a steel surface, or apolyolefin surface such as a polypropylene surface or a polyethylenesurface such as an ultra-high molecular weight polyethylene surface.

Other features and aspects of the present disclosure are discussed ingreater detail below.

DETAILED DESCRIPTION

Reference now will be made in detail to the embodiments of theinvention, one or more examples of which are set forth below. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment, can be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncover such modifications and variations.

In general, the present disclosure is directed to a polyoxymethylenepolymer composition and a polymer article comprising thepolyoxymethylene polymer composition with improved tribologicalproperties such as a reduced coefficient of friction. The tribologicalproperties can be improved by utilizing tribological modifiers. Ingeneral, the polyoxymethylene polymer composition comprises apolyoxymethylene polymer and at least one tribological modifier. Forinstance, the tribological modifier may include boron nitride,ultra-high molecular weight silicone, or a combination thereof.

The present inventors have discovered that by utilizing thepolyoxymethylene composition of the present invention, Improved slidingproperties and a reduced coefficient of friction against other surfacesmay be obtained. In particular, the composition and a polymer articlemade from the composition may exhibit a reduced coefficient of frictionagainst other surfaces or counter-materials while still exhibitingdesirable mechanical properties. In addition, these compositions andarticles also generate little frictional noise and experience low wear.

Polyoxymethylene Polymer

According to the present disclosure, the polyoxymethylene polymercomposition comprises a polyoxymethylene polymer.

The preparation of the polyoxymethylene polymer can be carried out bypolymerization of polyoxymethylene-forming monomers, such as trioxane ora mixture of trioxane and a cyclic acetal such as dioxolane in thepresence of ethylene glycol as a molecular weight regulator. Thepolyoxymethylene polymer used in the polymer composition may comprise ahomopolymer or a copolymer. According to one embodiment, thepolyoxymethylene is a homo- or copolymer which comprises at least 50mol. %, such as at least 75 mol. %, such as at least 90 mol. % and suchas even at least 97 mol. % of —CH₂O-repeat units.

In one embodiment, a polyoxymethylene copolymer is used. The copolymercan contain from about 0.1 mol. % to about 20 mol. % and in particularfrom about 0.5 mol. % to about 10 mol. % of repeat units that comprise asaturated or ethylenically unsaturated alkylene group having at least 2carbon atoms, or a cycloalkylene group, which has sulfur atoms or oxygenatoms in the chain and may include one or more substituents selectedfrom the group consisting of alkyl cycloalkyl, aryl, aralkyl,heteroaryl, halogen or alkoxy. In one embodiment, a cyclic ether oracetal is used that can be introduced into the copolymer via aring-opening reaction.

Preferred cyclic ethers or acetals are those of the formula:

in which x is 0 or 1 and R² is a C₂-C₄-alkylene group which, ifappropriate, has one or more substituents which are C₁-C₄-alkyl groups,or are C₁-C₄-alkoxy groups, and/or are halogen atoms, preferablychlorine atoms. Merely by way of example, mention may be made ofethylene oxide, propylene 1,2-oxide, butylene 1,2-oxide, butylene1,3-oxide, 1,3-dioxane, 1,3-dioxolane, and 1,3-dioxepan as cyclicethers, and also of linear oligo- or polyformals, such as polydioxolaneor polydioxepan, as comonomers.

It is particularly advantageous to use copolymers composed of from 99.5to 95 mol. % of trioxane and of from 0.5 to 5 mol. %, such as from 0.5to 4 mol. %, of one of the above-mentioned comonomers.

The polymerization can be effected as precipitation polymerization or inthe melt. By a suitable choice of the polymerization parameters, such asduration of polymerization or amount of molecular weight regulator, themolecular weight and hence the MVR value of the resulting polymer can beadjusted.

In one embodiment, a polyoxymethylene polymer with hydroxyl terminalgroups can be produced using a cationic polymerization process followedby solution hydrolysis to remove any unstable end groups. Duringcationic polymerization, a glycol, such as ethylene glycol can be usedas a chain terminating agent. The cationic polymerization results in abimodal molecular weight distribution containing low molecular weightconstituents. In one particular embodiment, the low molecular weightconstituents can be significantly reduced by conducting thepolymerization using a heteropoly acid such as phosphotungstic acid asthe catalyst. When using a heteropoly acid as the catalyst, forinstance, the amount of low molecular weight constituents can be lessthan about 2 wt. %.

A heteropoly acid refers to polyacids formed by the condensation ofdifferent kinds of oxo acids through dehydration and contains a mono- orpoly-nuclear complex ion wherein a hetero element is present in thecenter and the oxo acid residues are condensed through oxygen atoms.Such a heteropoly acid is represented by the formula:H_(x)[MmM′nOz]_(y)H₂Owherein

M represents an element selected from the group consisting of P, Si, Ge,Sn, As, Sb, U, Mn, Re, Cu, Ni, Ti, Co, Fe, Cr, Th or Ce,

M′ represents an element selected from the group consisting of W, Mo, Vor Nb,

m is 1 to 10,

n is 6 to 40,

z is 10 to 100,

x is an integer of 1 or above, and

y is 0 to 50.

The central element (M) in the formula described above may be composedof one or more kinds of elements selected from P and Si and thecoordinate element (M′) is composed of at least one element selectedfrom W, Mo and V, particularly W or Mo.

Specific examples of heteropoly acids are phosphomolybdic acid,phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdovanadicacid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid,silicotungstic acid, silicomolybdic acid, silicomolybdotungstic acid,silicomolybdotungstovanadic acid and acid salts thereof. Excellentresults have been achieved with heteropoly acids selected from12-molybdophosphoric acid (H₃PMo₁₂O₄₀) and 12-tungstophosphoric acid(H₃PW₁₂O₄₀) and mixtures thereof.

The heteropoly acid may be dissolved in an alkyl ester of a polybasiccarboxylic acid. It has been found that alkyl esters of polybasiccarboxylic acid are effective to dissolve the heteropoly acids or saltsthereof at room temperature (25° C.).

The alkyl ester of the polybasic carboxylic acid can easily be separatedfrom the production stream since no azeotropic mixtures are formed.Additionally, the alkyl ester of the polybasic carboxylic acid used todissolve the heteropoly acid or an acid salt thereof fulfills the safetyaspects and environmental aspects and, moreover, is inert under theconditions for the manufacturing of oxymethylene polymers.

Preferably the alkyl ester of a polybasic carboxylic acid is an alkylester of an aliphatic dicarboxylic acid of the formula:(ROOC)—(CH₂)n-(COOR′)wherein

n is an integer from 2 to 12, preferably 3 to 6 and

R and R′ represent independently from each other an alkyl group having 1to 4 carbon atoms, preferably selected from the group consisting ofmethyl, ethyl, n-propyl, iso-propyl, n-butyl, Iso-butyl and tert-butyl.

In one embodiment, the polybasic carboxylic acid comprises the dimethylor diethyl ester of the above-mentioned formula, such as a dimethyladipate (DMA).

The alkyl ester of the polybasic carboxylic acid may also be representedby the following formula:(ROOC)₂—CH—(CH₂)m-CH—(COOR′)₂wherein

m is an integer from 0 to 10, preferably from 2 to 4 and

R and R′ are independently from each other alkyl groups having 1 to 4carbon atoms, preferably selected from the group consisting of methyl,ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl and tert.-butyl.

Particularly preferred components which can be used to dissolve theheteropoly acid according to the above formula are butantetracarboxylicacid tetratethyl ester or butantetracarboxylic acid tetramethyl ester.

Specific examples of the alkyl ester of a polybasic carboxylic acid aredimethyl glutaric acid, dimethyl adipic acid, dimethyl pimelic acid,dimethyl suberic acid, diethyl glutaric acid, diethyl adipic acid,diethyl pimelic acid, diethyl suberic acid, dimethyl phthalic acid,dimethyl isophthalic acid, dimethyl terephthalic acid, diethyl phthalicacid, diethyl isophthalic acid, diethyl terephthalic acid,butantetracarboxylic acid tetramethylester and butantetracarboxylic acidtetraethylester as well as mixtures thereof. Other examples includedimethylisophthalate, diethylisophthalate, dimethylterephthalate ordiethylterephthalate.

Preferably, the heteropoly acid is dissolved in the alkyl ester of thepolybasic carboxylic acid in an amount lower than 5 wt. %, preferably inan amount ranging from 0.01 to 5 wt. %, wherein the weight is based onthe entire solution.

In some embodiments, the polymer composition of the present disclosuremay contain other polyoxymethylene homopolymers and/or polyoxymethylenecopolymers. Such polymers, for instance, are generally unbranched linearpolymers which contain at least 80%, such as at least 90%, oxymethyleneunits.

The polyoxymethylene polymer can have any suitable molecular weight. Themolecular weight of the polymer, for instance, can be from about 4,000grams per mole to about 20,000 g/mol. In other embodiments, however, themolecular weight can be well above 20,000 g/mol, such as from about20,000 g/mol to about 100,000 g/mol.

The polyoxymethylene polymer present in the composition can generallymelt flow index (MFI) ranging from about 1 to about 50 g/10 min, asdetermined according to ISO 1133 at 190° C. and 2.16 kg, thoughpolyoxymethylenes having a higher or lower melt flow index are alsoencompassed herein. For example, the polyoxymethylene polymer may be alow or mid-molecular weight polyoxymethylene that has a melt flow indexof greater than about 5 g/10 min, greater than about 10 g/10 min, orgreater than about 15 g/10 min. The melt flow index of thepolyoxymethylene polymer can be less than about 25 g/10 min, less thanabout 20 g/10 min, less than about 18 g/10 min, less than about 15 g/10min, less than about 13 g/10 min, or less than about 12 g/10 min. Thepolyoxymethylene polymer may for instance be a high molecular weightpolyoxymethylene that has a melt flow index of less than about 5 g/10min, less than about 3 g/10 min, or less than about 2 g/10 min.

The polyoxymethylene polymer may contain a relatively high amount offunctional groups, such as hydroxyl groups in the terminal positions.More particularly, the polyoxymethylene polymer can have terminalhydroxyl groups, for example hydroxyethylene groups and/or hydroxyl sidegroups, in at least more than about 50% of all the terminal sites on thepolymer. It should be understood that the total number of terminalgroups present includes all side terminal groups. In addition to theterminal hydroxyl groups, the polyoxymethylene polymer may also haveother terminal groups usual for these polymers such as alkoxy groups,formate groups, acetate groups or hemiacetal groups.

The polyoxymethylene polymer may also optionally have a relatively lowamount of low molecular weight constituents. As used herein, lowmolecular weight constituents (or fractions) refer to constituentshaving molecular weights below 10,000 dalton. In this regard, thepolyoxymethylene polymer can contain low molecular weight constituentsin an amount less than about 10 wt. %, based on the total weight of thepolyoxymethylene. In certain embodiments, for instance, thepolyoxymethylene polymer may contain low molecular weight constituentsin an amount less than about 5 wt. %, such as in an amount less thanabout 3 wt. %, such as even in an amount less than about 2 wt. %.

Suitable commercially available polyoxymethylene polymers are availableunder the trade name Hostaform® (HF) by Celanese/Ticona.

The polyoxymethylene polymer may be present in the polyoxymethylenepolymer composition in an amount of at least 60 wt. %, such as at least70 wt. %, such as at least 80 wt. %, such as at least 85 wt. %, such asat least 90 wt. %, such as at least 95 wt. %. In general, thepolyoxymethylene polymer is present in an amount of less than about 100wt. %, such as less than about 99 wt. %, such as less than about 97 wt.%, wherein the weight is based on the total weight of thepolyoxymethylene polymer composition.

Tribological Modifiers

According to the present disclosure, the polyoxymethylene polymercomposition and the polymer article comprising the polyoxymethylenepolymer composition may comprise at least one tribological modifier. Forinstance, the polyoxymethylene polymer composition may comprise boronnitride, ultra-high molecular weight silicone, or a combination thereof.

In one embodiment, boron nitride may be used to modify thepolyoxymethylene polymer. Boron nitride can be particularly beneficialin improving the tribological properties and reducing the coefficient offriction of polyoxymethylene. Boron nitride exists in a variety ofdifferent crystalline forms (e.g., h-BN—hexagonal, c-BN—cubic orspharlerite, and w-BN—wurtzite). In one embodiment, hexagonal boronnitride may be used in the composition. Not to be limited by theory, theh-BN may promote lubricity due to its layered structure and weaksecondary forces between adjacent layers allowing or easy sliding of thelayers. The boron nitride may have an average particle size ranging fromabout 0.5 μm to about 10 μm, such as from about 1 μm to about 6 μm, suchas about 1.5 μm or 5 μm. The boron nitride may be present in thepolyoxymethylene polymer composition in an amount of at least about 0.1wt. %, such as at least about 0.5 wt. %, such as at least about 0.75 wt.%, such as at least about 1 wt. %, such as at least about 2 wt. % andgenerally less than about 10 wt. %, such as less than about 5 wt. %,such as less than about 2.5 wt. %, such as less than about 2 wt. %,wherein the weight is based on the total weight of the polyoxymethylenepolymer composition. In one embodiment, the composition may besubstantially free of the boron nitride such that it is present in anamount of 0 wt. %.

In another embodiment, ultra-high molecular weight silicone (UHMW-Si)may be used to modify the polyoxymethylene polymer. In general, theUHMW-Si can have an average molecular weight of greater than 100,000g/mol, such as greater than about 200,000 g/mol, such as greater thanabout 300,000 g/mol, such as greater than about 500,000 g/mol and lessthan about 3,000,000 g/mol, such as less than about 2,000,000 g/mol,such as less than about 1,000,000 g/mol, such as less than about 500,000g/mol, such as less than about 300,000 g/mol. Generally, the UHMW-Si canhave a kinematic viscosity at 40° C. measured according to DIN 51562 ofgreater than 100,000 mm² s⁻¹, such as greater than about 200,000 mm²s⁻¹, such as greater than about 1,000,000 mm² s⁻¹, such as greater thanabout 5,000,000 mm² s⁻¹, such as greater than about 10,000,000 mm² s⁻¹,such as greater than about 15,000,000 mm² s⁻¹ and less than about50,000,000 mm² s⁻¹, such as less than about 25,000,000 mm² s⁻¹, such asless than about 10,000,000 mm² s⁻¹, such as less than about 1,000,000mm² s⁻¹, such as less than about 500,000 mm² s⁻¹, such as less thanabout 200,000 mm² s⁻¹.

The UHMW-Si may comprise a siloxane such as a polysiloxane orpolyorganosiloxane. In one embodiment, the UHMW-Si may comprise adialkylpolysiloxane such as a dimethylsiloxane, an alkylarylsiloxanesuch as a phenylmethylsilaoxane, or a diarylsiloxane such as adiphenylsiloxane, or a homopolymer thereof such as apolydimethylsiloxane or a polymethylphenylsiloxane, or a copolymerthereof with the above molecular weight and/or kinematic viscosityrequirements. The polysiloxane or polyorganosiloxane may also bemodified with a substituent such as an epoxy group, a hydroxyl group, acarboxyl group, an amino group or a substituted amino group, an ethergroup, or a meth(acryloyl) group in the end or main chain of themolecule. The UHMW-Si compounds may be used singly or in combination.Any of the above UHMW-Si compounds may be used with the above molecularweight and/or kinematic viscosity requirements.

The UHMW-Si may be added to the polyoxymethylene polymer composition asa masterbatch wherein the UHMW-Si is dispersed in a polyoxymethylenepolymer and the masterbatch is thereafter added to anotherpolyoxymethylene polymer. The masterbatch may comprise from about 10 wt.% to about 50 wt. %, such as from about 35 wt. % to about 45 wt. %, suchas about 40 wt. % of an UHMW-Si.

The UHMW-Si may be present in the polyoxymethylene polymer compositionin an amount of at greater than about 0 wt. %, such as at greater thanabout 0.1 wt. %, such as at greater than about 0.5 wt. %, such as atgreater than about 0.75 wt. %, such as at greater than about 1 wt. %,such as at greater than about 2 wt. %, such as at greater than about 2.5wt. % and generally less than about 10 wt. %, such as less than about 6wt. %, such as less than about 5 wt. %, such as less than about 4 wt. %,such as less than about 3.5 wt. %, such as less than about 3 wt. %,wherein the weight is based on the total weight of the polyoxymethylenepolymer composition.

According to another embodiment, boron nitride, such as hexagonal-boronnitride, and UHMW-Si may be utilized in combination to modify thepolyoxymethylene polymer. The present inventors have discovered thatwhen both tribological modifiers are used simultaneously, thecombination can provide a synergistic effect with a resulting polymercomposition that exhibits improved tribological properties whilemaintaining or even improving the mechanical properties. In suchembodiments, the boron nitride and UHMW-Si may be utilized in thepolyoxymethylene polymer composition in the amounts disclosed above.

According to another embodiment, UHMW-Si may be utilized in combinationwith PTFE. The present inventors have discovered that when bothtribological modifiers are used simultaneously, the combination canprovide a synergistic effect with a resulting polymer composition thatexhibits improved tribological properties while maintaining or evenimproving the mechanical properties. In one embodiment, the PTFE may bein the form of a powder. In another embodiment, the PTFE may be in theform of a fiber. When used in combination with UHMW-Si, in oneembodiment, the amount of PTFE may be present in an amount of at least0.1 wt. %, such as at least 1 wt. %, such as at least 5 wt. %, such asat least 10 wt. %, such as at least 15 wt. % and generally less thanabout 50 wt. %, such as less than about 40 wt. %, such as less thanabout 30 wt. %, such as less than about 25 wt. %, such as less thanabout 15 wt. %, such as less than about 10 wt. %. The reduction in PTFEmay still provide a composition with desired tribological properties.The PTFE and UHMW-Si may be utilized in the polyoxymethylene polymercomposition in the amounts disclosed above.

According to the present disclosure, various other tribologicalmodifiers may be incorporated into the polyoxymethylene polymercomposition. These tribological modifiers may include, for instance,calcium carbonate particles, ultrahigh-molecular-weight polyethylene(UHMW-PE) particles, stearyl stearate particles, silicone oil, apolyethylene wax, an amide wax, wax particles comprising an aliphaticester wax comprised of a fatty acid and a monohydric alcohol, a graftcopolymer with an olefin polymer as a graft base, or a combinationthereof. These tribological modifiers include the following:

(1) From 0.1-50 wt. %, such as from 1-25 wt. %, of a calcium carbonateparticle such as a calcium carbonate (chalk) powder.

(2) From 0.1-50 wt. %, such as from 1-25 wt. %, such as from 2.5-20 wt.%, such as from 5 to 15 wt. %, of an ultrahigh-molecular-weightpolyethylene (UHMW-PE) powder. UHMW-PE can be employed as a powder, inparticular as a micro-powder. The UHMW-PE generally has a mean particlediameter D₅₀ (volume based and determined by light scattering) in therange of 1 to 5000 μm, preferably from 10 to 500 μm, and particularlypreferably from 10 to 150 μm such as from 30 to 130 μm, such as from 80to 150 μm, such as from 30 to 90 μm.

The UHMW-PE can have an average molecular weight of higher than 1.0·10⁶g/mol, such as higher than 2.0·10⁶ g/mol, such as higher than 4.0·10⁶g/mol, such as ranging from 1.0·10⁶ g/mol to 15.0·10⁶ g/mol, such asfrom 3.0·10⁶ g/mol to 12.0·10⁶ g/mol, determined by viscosimetry.Preferably, the viscosity number of the UHMW-PE is higher than 1000ml/g, such as higher than 1500 ml/g, such as ranging from 1800 ml/g to5000 ml/g, such as ranging from 2000 ml/g to 4300 ml/g (determinedaccording to ISO 1628, part 3; concentration in decahydronaphthalin:0.0002 g/ml).

(3) From 0.1-10 wt. %, such as from 0.1-5 wt. %, such as from 0.5-3 wt.%, of stearyl stearate.

(4) From 0.1-10 wt. %, such as from 0.5-5 wt. %, such as from 0.8-2 wt.%, of a silicone oil. Alternatively, in one embodiment, the compositionmay be substantially free of silicone oil, such that the silicone oil ispresent in an amount of less than about 0.1 wt. %, such as less thanabout 0.05 wt. %, such as less than about 0.01 wt. %, such as about 0wt. %. In another embodiment, the composition may not comprise acombination of silicone oil and UHMW-Si alone. In such embodiments, thecomposition may comprise UHMW-Si, silicone oil, and another tribologicalmodifier, such as a boron nitride or PTFE.

When silicone oil is present in the composition, the silicone oil canhave an average molecular weight of at least about 5,000 g/mol, such asat least about 20,000 g/mol, such as at least about 50,000 g/mol andgenerally less than 100,000 g/mol, such as less than about 75,000 g/mol,such as less than about 50,000 g/mol. The silicone oil can have akinematic viscosity at 40° C. measured according to DIN 51562 of greaterthan about 100 mm² s⁻¹, such as greater than about 5,000 mm² s⁻¹, suchas greater than about 15,000 mm² s⁻¹ and generally less than 100,000 mm²s⁻¹, such as less than about 50,000 mm² s⁻¹, such as less than about25,000 mm² s⁻¹, such as less than about 15,000 mm² s⁻¹. The silicone oilmay comprise a liquid polysiloxane such as a polydimethylsiloxane at aroom temperature of 25° C. with the above molecular weight and/orkinematic viscosity specifications.

(5) From 0.1-5 wt. %, such as from 0.5-3 wt. %, of a polyethylene wax,such as an oxidized polyethylene wax.

(6) From 0.1-5 wt. %, such as from 0.2-2 wt. %, of an amide wax.

(7) From 0.1-5 wt. %, such as from 0.5-3 wt. %, of an aliphatic esterwax composed of a fatty acid and of a monohydric alcohol.

(8) From 0.1-50 wt. %, such as from 1-25 wt. %, such as from 2-10 wt. %by weight of a graft copolymer which has an olefin polymer as a graftbase and, grafted on this, at least one vinyl polymer or one etherpolymer, and/or a graft copolymer which has an elastomeric core based onpolydienes and a hard graft envelope composed of (meth)acrylates and/orof (meth)acrylonitriles. A suitable graft base can be any olefinhomopolymer (e.g., polyethylene or polypropylene) or copolymer orcopolymers derived from copolymerizable ethylenically unsaturatedmonomers (e.g, ethylenepropylene copolymers, ethylene-1-butenecopolymers, ethylene/glycidyl (meth)acrylate copolymers). Suitable graftmonomers are any of the ethylenically unsaturated monomers having apolar group or other graftable monomers having polar groups that modifythe polarity of the essentially non-polar graft base (e.g. ethylenicallyunsaturated carboxylic acids such as (meth)acrylic acid and derivativesthereof in combination with acrylonitrile or styrene/acrylonitrile, ifappropriate). In one embodiment, the graft copolymer may comprise apolyethylene or polypropylene graft base grafted with acrylonitrile orwith styrene/acrylonitrile.

In general, the tribological modifiers improve the tribologicalproperties of the polyoxymethylene polymer composition by reducing thecoefficient of friction and wear when contacted with another surface orcounter-material. In addition, in some instances, the tribologicalmodifiers may even improve the mechanical properties of thepolyoxymethylene polymer composition and a polymer article producedtherefrom.

According to the present disclosure, tribological modifiers improve thetribological properties of the polyoxymethylene polymer compositions andpolymer articles produced therefrom without the need for an externallubricant, such as water-based or PTFE-based external lubricants, whenutilized in tribological applications. An external lubricant may be alubricant that is applied to a polymer article or polyoxymethylene basedsystem of the present disclosure. In one embodiment, an externallubricant may not be associated with the polyoxymethylene polymercomposition or polymer article such that the external lubricant is notpresent on a surface of the polyoxymethylene polymer composition orpolymer article. In another embodiment, an external lubricant may beutilized with the polyoxymethylene polymer composition and polymerarticle of the present disclosure.

Other Additives

The polymer composition of the present disclosure may also contain otherknown additives such as, for example, antioxidants, formaldehydescavengers, acid scavengers, UV stabilizers or heat stabilizers,reinforcing fibers. In addition, the compositions can contain processingauxiliaries, for example adhesion promoters, lubricants, nucleants,demolding agents, fillers, or antistatic agents and additives whichimpart a desired property to the compositions and articles or partsproduced therefrom.

In one embodiment, an ultraviolet light stabilizer may be present. Theultraviolet light stabilizer may comprise a benzophenone, abenzotriazole, or a benzoate. The UV light absorber, when present, maybe present in the polymer composition in an amount of at least about0.01 wt. %, such as at least about 0.05 wt. %, such as at least about0.075 wt. % and less than about 1 wt. %, such as less than about 0.75wt. %, such as less than about 0.5 wt. %, wherein the weight is based onthe total weight of the respective polymer composition.

In one embodiment, a formaldehyde scavenger, such as anitrogen-containing compound, may be present. Mainly, of these areheterocyclic compounds having at least one nitrogen atom as hetero atomwhich is either adjacent to an amino-substituted carbon atom or to acarbonyl group, for example pyridine, pyrimidine, pyrazine, pyrrolidone,aminopyridine and compounds derived therefrom. Other particularlyadvantageous compounds are triamino-1,3,5-triazine (melamine) and itsderivatives, such as melamine-formaldehyde condensates and methylolmelamine. Oligomeric polyamides are also suitable in principle for useas formaldehyde scavengers. The formaldehyde scavenger may be usedindividually or in combination.

Further, the formaldehyde scavenger may be a guanamine compound whichmay include an aliphatic guanamine-based compound, an alicyclicguanamine-based compound, an aromatic guanamine-based compound, a heteroatom-containing guanamine-based compound, or the like. The formaldehydescavenger may be present in the polymer composition in an amount of atleast about 0.01 wt. %, such as at least about 0.05 wt. %, such as atleast about 0.075 wt. % and less than about 1 wt. %, such as less thanabout 0.75 wt. %, such as less than about 0.5 wt. %, wherein the weightis based on the total weight of the respective polymer composition.

In one embodiment, an acid scavenger may be present. The acid scavengermay comprise, for instance, an alkaline earth metal salt. For instance,the acid scavenger may comprise a calcium salt, such as a calciumcitrate. The acid scavenger may be present in an amount of at leastabout 0.001 wt. %, such as at least about 0.005 wt. %, such as at leastabout 0.0075 wt. % and less than about 1 wt. %, such as less than about0.75 wt. %, such as less than about 0.5 wt. %, wherein the weight isbased on the total weight of the respective polymer composition.

In one embodiment, a nucleant may be present. The nucleant may increasecrystallinity and may comprise an oxymethylene terpolymer. In oneparticular embodiment, for instance, the nucleant may comprise aterpolymer of butanediol diglycidyl ether, ethylene oxide, and trioxane.The nucleant may be present in the composition in an amount of at leastabout 0.01 wt. %, such as at least about 0.05 wt. %, such as at leastabout 0.1 wt. % and less than about 2 wt. %, such as less than about 1.5wt. %, such as less than about 1 wt. %, wherein the weight is based onthe total weight of the respective polymer composition.

In one embodiment, an antioxidant, such as a sterically hindered phenol,may be present. Examples which are available commercially, arepentaerythrityltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], triethyleneglycol bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate],3,3′-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionohydrazide], andhexamethylene glycolbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. The antioxidantmay be present in the polymer composition in an amount of at least about0.01 wt. %, such as at least about 0.05 wt. %, such as at least about0.075 wt. % and less than about 1 wt. %, such as less than about 0.75wt. %, such as less than about 0.5 wt. %, wherein the weight is based onthe total weight of the respective polymer composition.

In one embodiment, a light stabilizer, such as a sterically hinderedamine, may be present in addition to the ultraviolet light stabilizer.Hindered amine light stabilizers that may be used include oligomerichindered amine compounds that are N-methylated. For instance, hinderedamine light stabilizer may comprise a high molecular weight hinderedamine stabilizer. The light stabilizers, when present, may be present inthe polymer composition in an amount of at least about 0.01 wt. %, suchas at least about 0.05 wt. %, such as at least about 0.075 wt. % andless than about 1 wt. %, such as less than about 0.75 wt. %, such asless than about 0.5 wt. %, wherein the weight is based on the totalweight of the respective polymer composition.

In one embodiment, a lubricant, not including the tribological modifiersmentioned above, may be present. The lubricant may comprise a polymerwax composition. Further, in one embodiment, a polyethylene glycolpolymer (processing aid) may be present in the composition. Thepolyethylene glycol, for instance, may have a molecular weight of fromabout 1000 to about 5000, such as from about 3000 to about 4000. In oneembodiment, for instance, PEG-75 may be present. In another embodiment,a fatty acid amide such as ethylene bis(stearamide) may be present.Lubricants may generally be present in the polymer composition in anamount of at least about 0.01 wt. %, such as at least about 0.05 wt. %,such as at least about 0.075 wt. % and less than about 1 wt. %, such asless than about 0.75 wt. %, such as less than about 0.5 wt. %, whereinthe weight is based on the total weight of the respective polymercomposition.

In one embodiment, a compatibilizer, such as a phenoxy resin, may bepresent. Generally, the phenoxy resin may be present in the compositionin an amount of at least about 0.01 wt. %, such as at least about 0.05wt. %, such as at least about 0.075 wt. % and less than about 1 wt. %,such as less than about 0.75 wt. %, such as less than about 0.5 wt. %,wherein the weight is based on the total weight of the respectivepolymer composition.

In one embodiment, a colorant may be present. Colorants that may be usedinclude any desired Inorganic pigments, such as titanium dioxide,ultramarine blue, cobalt blue, and other organic pigments and dyes, suchas phthalocyanines, anthraquinnones, and the like. Other colorantsinclude carbon black or various other polymer-soluble dyes. The colorantmay be present in the composition in an amount of at least about 0.01wt. %, such as at least about 0.05 wt. %, such as at least about 0.1 wt.% and less than about 5 wt. %, such as less than about 2.5 wt. %, suchas less than about 1 wt. %, wherein the weight is based on the totalweight of the respective polymer composition.

In one embodiment, a coupling agent may be present. Coupling agents usedinclude polyfunctional coupling agents, such as trifunctional orbifunctional agents. A suitable coupling agent is a polyisocyanate suchas a diisocyanate. The coupling agent may provide a linkage between thepolyoxymethylene polymer and the reinforcing fiber and/or sizingmaterial coated on the reinforcing fiber. Generally, the coupling agentis present in an amount of at least about 0.1 wt. %, such as at leastabout 0.2 wt. % such as at least about 0.3 wt. % and less than about 5wt. %, such as less than about 3 wt. %, such as less than about 1.5 wt.%. Alternatively, the composition may also be substantially free of anycoupling agents such as less than about 0.2 wt. %, such as less thanabout 0.1 wt. %, such as less than about 0.05 wt. %, such as less thanabout 0.01 wt. %, such as about 0 wt. %.

In one embodiment, a reinforcing fiber may be present. The reinforcingfibers which may be used according to the present invention includemineral fibers, glass fibers, polymer fibers such as aramid fibers,metal fibers such as steel fibers, carbon fibers, or natural fibers.These fibers may be unmodified or modified, e.g. provided with a sizingor chemically treated, in order to improve adhesion to the polymer.Fiber diameters can vary depending upon the particular fiber used andwhether the fiber is in either a chopped or a continuous form. Thefibers, for instance, can have a diameter of from about 5 μm to about100 μm, such as from about 5 μm to about 50 μm, such as from about 5 μmto about 15 μm. When present, the respective composition may containreinforcing fibers in an amount of at least 1 wt. %, such as at least 5wt. %, such as at least 7 wt. %, such as at least 10 wt. %, such as atleast 15 wt. % and generally less than about 50 wt. %, such as less thanabout 45 wt. %, such as less than about 40 wt. %, such as less thanabout 30 wt. %, such as less than about 20 wt. %, wherein the weight isbased on the total weight of the respective polyoxymethylene polymercomposition. Alternatively, the polyoxymethylene polymer composition mayalso be substantially free of any reinforcing fibers, such that thecomposition contains fibers in an amount of less than about 0.1 wt. %,such as less than about 0.05 wt. %, such as less than about 0.01 wt. %,such as about 0 wt. %.

Polymer Articles

The compositions of the present disclosure can be compounded and formedinto a polymer article using any technique known in the art. Forinstance, the respective composition can be intensively mixed to form asubstantially homogeneous blend. The blend can be melt kneaded at anelevated temperature, such as a temperature that is higher than themelting point of the polymer utilized in the polymer composition butlower than the degradation temperature. Alternatively, the respectivecomposition can be melted and mixed together in a conventional single ortwin screw extruder. Preferably, the melt mixing is carried out at atemperature ranging from 100 to 280° C., such as from 120 to 260° C.,such as from 140 to 240° C. or 180 to 220° C. However, such processingshould be conducted for each respective composition at a desiredtemperature to minimize any polymer degradation.

After extrusion, the compositions may be formed into pellets. Thepellets can be molded into polymer articles by techniques known in theart such as injection molding, thermoforming, blow molding, rotationalmolding and the like. According to the present disclosure, the polymerarticles demonstrate excellent tribological behavior and mechanicalproperties. Consequently, the polymer articles can be used for severalapplications where low wear and excellent gliding properties aredesired.

Polymer articles include any moving articles or moldings that are incontact with another surface and may require high tribologicalrequirements. For instance, polymer articles include articles for theautomotive industry, especially housings, latches such as rotarylatches, window winding systems, wiper systems, pulleys, sun roofsystems, seat adjustments, levers, bushes, gears, gear boxes, claws,pivot housings, wiper arms, brackets or seat rail bearings, zippers,switches, cams, rollers or rolling guides, sliding elements or glidessuch as sliding plates, conveyor belt parts such as chain elements andlinks, castors, fasteners, levers, conveyor system wear strips and guardrails, medical equipment such as medical inhalers and injectors. Analmost limitless variety of polymer articles may be formed from thepolymer compositions of the present disclosure.

Properties

Utilizing the polyoxymethylene polymer composition and polymer articleproduced therefrom according to the present disclosure providescompositions and articles with improved tribological properties.According to the present disclosure, the tribological properties aregenerally measured by the coefficient of friction.

In general, static friction is the friction between two or more surfacesthat are not moving relative to each other (ie., both objects arestationary). In general, dynamic friction occurs when two objects aremoving relative to each other (ie., at least one object is in motion orrepeated back and forth motion). In addition, stick-slip is generallyknown as a phenomenon caused by continuous alternating between staticand dynamic friction.

According to the present disclosure, the composition and polymer articlemay exhibit a static coefficient of friction against another surface, asdetermined according to VDA 230-206, of greater than about 0.01, such asgreater than about 0.02, such as greater than about 0.03, such asgreater than about 0.04, such as greater than about 0.05, such asgreater than about 0.06 and generally less than about 0.2, such as lessthan about 0.18, such as less than about 0.15, such as less than about0.12, such as less than about 0.1, such as less than about 0.9, such asless than about 0.8, such as less than about 0.7, such as less thanabout 0.6.

According to the present disclosure, the composition and polymer articlemay exhibit a dynamic coefficient of friction against another surface,as determined according to VDA 230-206, of greater than about 0.01, suchas greater than about 0.02, such as greater than about 0.03, such asgreater than about 0.04, such as greater than about 0.05, such asgreater than about 0.06 and generally less than about 0.2, such as lessthan about 0.18, such as less than about 0.15, such as less than about0.12, such as less than about 0.1, such as less than about 0.9, such asless than about 0.8, such as less than about 0.7, such as less thanabout 0.6.

In addition, the effect of sliding speed on the dynamic coefficient offriction was also measured at speeds of 0.1 mm/s, 1 mm/s, 10 mm/s, and100 mm/s. This test method utilizes a ball-on-prism configuration wherea ball made of a plastic or metal rotates uni-directionally against theplate of a counter material. At a speed of 0.1 mm/s, the dynamiccoefficient of friction against another surface is greater than about0.02, such as greater than about 0.03, such as greater than about 0.04,such as greater than about 0.05 and generally less than about 0.1, suchas less than about 0.08, such as less than about 0.07, such as less thanabout 0.06. At a speed of 1 mm/s, the dynamic coefficient of frictionagainst another surface is greater than about 0.02, such as greater thanabout 0.03, such as greater than about 0.04, such as greater than about0.05 generally less than about 0.1, such as less than about 0.08, suchas less than about 0.07, such as less than about 0.06. At a speed of 10mm/s, the dynamic coefficient of friction against another surface isgreater than about 0.03, such as greater than about 0.04, such asgreater than about 0.05, such as greater than about 0.06 and generallyless than about 0.15, such as less than about 0.12, such as less thanabout 0.1, such as less than about 0.09, such as less than about 0.08,such as less than about 0.07. At a speed of 100 mm/s, the dynamiccoefficient of friction against another surface is greater than about0.05, such as greater than about 0.07, such as greater than about 0.08,such as greater than about 0.09 and generally less than about 0.17, suchas less than about 0.15, such as less than about 0.12, such as less thanabout 0.1, such as less than about 0.09, such as less than about 0.08.

In one embodiment, the above static coefficient of friction and dynamiccoefficient of friction values and effect of sliding speed on thedynamic coefficient of friction are exhibited between the composition orpolymer article and various counter-materials. For instance, the abovevalues may be exhibited between the composition or polymer article and apolyester surface such as a polyethylene terephthalate surface. Inanother embodiment, the above values may be exhibited between thecomposition or polymer article and a polyacetal surface, a metal surfacesuch as a steel surface, or a polyolefin surface such as a polypropylenesurface or a polyethylene surface such as an ultra-high molecular weightpolyethylene surface.

In one embodiment, the composition and articles of the presentdisclosure may exhibit improved wear properties when compared topolyoxymethylene compositions and articles that are not modified with atribological modifier. The wear tests may be conducted utilizing a steelshaft, a shaft diameter of 65 mm, a roughness of 0.8 μm, a load of 3.1N, a sliding velocity of 136 m/min, a test duration of 60 h, and adistance of 490 km. The compositions of the present disclosure mayexhibit at least 20% reduced wear, such as at least 40% reduced wear,such as at least 50% reduced wear, such as at least 60% reduced wear,such as at least 80% reduced wear and less than about 100% reduced wear,such as less than about 90% reduced wear, such as less than about 80%reduced wear, when compared to the wear of a polyoxymethylene polymerthat is not modified with a tribological modifier.

While the polyoxymethylene polymer composition and polymer articlesproduced therefrom of the present invention provide improvedtribological properties, the compositions and articles may also exhibitimproved mechanical properties. For instance, the modulus of elasticity,determined according to ISO Test No. 527, of the composition or polymerarticle may be greater than about 2000 MPa, such as greater than about2200 MPa, such as greater than about 2400 MPa, such as greater thanabout 2500 MPa, such as greater than about 2600 MPa and generally lessthan about 10000 MPa, such as less than about 7500 MPa, such as lessthan about 5000 MPa, such as less than about 4000 MPa, such as less thanabout 3500 MPa, such as less than about 3000 MPa.

In one embodiment, the polymer article or molded polymer article mayhave topographical features that may provide surface characteristicsand/or surface roughness on at least one surface of the article. Forinstance, the features may be ridges, valleys, protrusions, and the likeon the surface of the article. These features may be present at thenanoscale or microscale level. Not to be limited by theory, the surfaceroughness may be produced during the molding of specific polymerarticles. Surface roughness may also be produced depending on theparticular additives present in the composition.

When in contact with a counter material, the surface roughness of thearticle may contribute to a reduced dynamic coefficient of friction whencompared to the dynamic coefficient of friction of an article thatexhibits a lesser degree of surface roughness. For instance, the dynamiccoefficient of friction of an article exhibiting surface roughness maybe less than the dynamic coefficient of friction of an article that issubstantially free of surface roughness.

The surface roughness depth (Rz) may be measured according to DIN 4768using a profilometer or roughness tester. The average surface roughnessdepth represents the mean from the Individual depths of roughness offive individual lines. For instance, the measurements are made betweenthe highest and lowest points on the surface averaged over fiveindividual lengths.

In one embodiment, the article produced according to the presentdisclosure may have an average surface area roughness of greater thanabout 0.1 μm, such as greater than about 0.25 μm, such as greater thanabout 0.50 μm, such as greater than about 1 μm, such as greater thanabout 2.5 μm, such as greater than about 5 μm and less than about 30 μm,such as less than about 20 μm, such as less than about 15 μm, such asless than about 10 μm, such as less than about 5 μm, such as less thanabout 2.5 μm, such as less than about 1 μm.

The present disclosure may be better understood with reference to thefollowing examples.

EXAMPLES

The examples of the invention are given below by way of illustration andnot by way of limitation. The following experiments were conducted inorder to show some of the benefits and advantages of the presentinvention.

Various polymer compositions comprising a polyoxymethylene polymer andat least one tribological modifier were produced in accordance with thepresent disclosure. In particular, the tribological modifiers includedhexagonal-boron nitride, ultra-high molecular weight silicone,ultra-high molecular weight polyethylene wax, silicone oil, andpolytetrafluoroethylene.

The components of each respective composition were mixed together andcompounded using a ZSK 25MC (Werner & Pfleiderer, Germany) twin screwextruder (zone temperature 190° C., melt temperature about 210*C). Thescrew configuration with kneading elements was chosen so that effectivethorough mixing of the components took place. The compositions wereextruded and pelletized. The pellets were dried for 8 hours at 120° C.and then injection molded.

The compositions/molds were tested for a variety of tribological andphysical properties. The results are provided in the tables below:

Example 1

In this example, the tribological properties (coefficient of frictionand effect of sliding speed on the dynamic coefficient of friction) weredetermined between the polyoxymethylene composition and a polyethyleneterephthalate surface.

Stick-slip tests were conducted to determine the dynamic coefficient offriction and the static coefficient of friction. Stick-slip tests wereconducted according to VDA 230-206. A ball-on-plate configuration wasutilized with a load of 12.5 N, sliding speed of 8 mm/s, and a testduration of 8 minutes.

Using a ball-on-prism test, the sliding speed was adjusted to determinethe effect on the dynamic coefficient of friction. The sliding speedeffect on the dynamic coefficient of friction was determined by applyinga load of 10 N and then increasing the sliding speed gradually from 0 to500 mm/s. The dynamic coefficient of friction was determined as afunction of sliding speed.

Samples 1-4 exhibited a higher degree of surface roughness in comparisonto Samples 5-10.

Sample Sample Sample Sample 1 2 3 4 POM copolymer (wt. %) (HF C9021)95.2 96.2 97.2 98 h-BN (wt. %) 1.0 1.0 0 2.0 UHMW-Si (wt. %) 2.8 2.8 2.80 Wax (wt. %) 1.0 0 0 0 Silicone oil (wt. %) 0 0 0 0 PTFE (wt. %) 0 0 00 Stick-slip test Dynamic CoF 0.044 0.040 0.042 0.045 Static CoF 0.0520.047 0.052 0.050 Sliding speed  0.1 mm/s 0.036 0.058 0.052 0.056 effecton dynamic  1 mm/s 0.037 0.054 0.045 0.043 coefficient of  10 mm/s 0.0610.056 0.047 0.071 friction 100 mm/s 0.096 0.099 0.080 0.055 SampleSample Sample Sample Sample Sample 5 6 7 8 9 10 POM copolymer (wt. %)(HF C9021) 99.5 99.0 98.0 98.4 98 97.2 h-BN (wt. %) 0.5 1.0 2.0 0 0 0UHMW-Si (wt. %) 0 0 0 1.6 2.0 2.8 Wax (wt. %) 0 0 0 0 0 0 Silicone oil(wt. %) 0 0 0 0 0 0 PTFE (wt. %) 0 0 0 0 0 0 Stick-slip test Dynamic CoF0.111 0.115 0.081 0.093 0.095 0.079 Static CoF 0.127 0.127 0.091 0.1040.108 0.095 Sliding speed  0.1 mm/s 0.117 0.074 0.073 0.037 0.025 0.028effect on dynamic  1 mm/s 0.087 0.078 0.075 0.035 0.033 0.031coefficient of  10 mm/s 0.103 0.084 0.074 0.067 0.063 0.042 friction 100mm/s 0.148 0.079 0.079 0.119 0.108 0.075 Sample Sample Sample 11 12 13POM copolymer (wt. %) (HF C9021) 95.2 96.9 80.35 h-BN (wt. %) 1.0 1.0 0UHMW-Si (wt. %) 2.8 1.6 3.0 Wax (wt. %) 0 0 0 Silicone oil (wt. %) 1.00.5 0 PTFE (wt. %) 0 0 16.65 Stick-slip test Dynamic CoF 0.085 0.0770.068 Static CoF 0.098 0.087 0.085 Sliding speed  0.1 mm/s 0.024 0.0510.026 effect on dynamic  1 mm/s 0.028 0.061 0.033 coefficient of  10mm/s 0.040 0.113 0.044 friction 100 mm/s 0.094 0.147 0.071

Example 2

The modulus of elasticity was tested according to ISO Test No. 527(technically equivalent to ASTM D 638). Modulus measurements wereconducted at a temperature of 23° C. using a dumbbell shaped specimenwith a length of about 165 mm, thickness of about 4 mm, and width in thegage area of about 10 mm.

Comparative Sample Sample Sample Sample Sample Sample Sample 1 14 15 1617 18 19 POM copolymer 100 99.5 99.0 98.0 98.8 98.4 98.0 (co-monomercontent of 3.4%) (wt. %) (HF C9021) POM copolymer 0 0 0 0 0 0 0(co-monomer content of 1.4%) (wt. %) (HF C13031) POM copolymer 0 0 0 0 00 0 (co-monomer content of 0.7%) (wt. %) (HF HS90) h-RN (wt. %) 0 0.51.0 2.0 0 0 0 UHMW-Si (wt. %) 0 0 0 0 1.2 1.6 2.0 Modulus of 2822 28492855 2879 2683 2621 2598 Elasticity (MPa) Sample Sample Sample SampleSample Sample 20 21 22 23 24 25 POM copolymer 97.2 96.2 0 0 0 0(co-monomer content of 3.4%) (wt. %) (HF C9021) POM copolymer 0 0 97.296.2 0 0 (co-monomer content of 1.4%) (wt. %) (HF C13031) POM copolymer0 0 0 0 97.2 96.2 (co-monomer content of 0.7%) (wt. %) (HF HS90) h-BN(wt. %) 0 1.0 0 1.0 0 1.0 UHMW-Si (wt. %) 2.8 2.8 2.8 2.8 2.8 2.8Modulus of 2544 2770 2712 2933 2880 3124 Elasticity (MPa)

Example 3

Stick-slip tests were conducted to compare a new mold and a tested orused mold. New molds were tested after injection molding. Used moldswere produced by conveying materials on new molds for a certain periodof time to simulate a used mold.

Stick-slip tests were conducted to determine the dynamic coefficient offriction and the static coefficient of friction. Stick-slip tests wereconducted according to VDA 230-206. A ball-on-plate configuration wasutilized with a load of 12.5 N, sliding speed of 8 mm/s, and a testduration of 8 minutes.

In this example, the tribological properties (coefficient of frictionand effect of sliding speed on the dynamic coefficient of friction) weredetermined between the polyoxymethylene composition and a polyethyleneterephthalate surface.

Sample Sample Sample Sample Sample Sample 26 27 28 29 30 31 POMcopolymer — — 97.2 97.2 — — (co-monomer content of 3.4%) (wt. %) (HFC9021) POM copolymer 98.5  97.8  — — 93 93 (co-monomer content of 1.4%)(wt. %) (HF C13031) PE Wax (wt. %) 1.0 1.0 — — — — PTFE (wt. %) 0.5 0.51.8 1.8 — — Silicone oil (wt. %) — — 1.0 1.0 2 2 Modifier — — — — 5 5New or Used New Used New Used New Used Stick-slip Dynamic CoF 0.06 0.060.06 0.07 0.07 0.09 test Static CoF 0.07 0.07 0.07 0.08 0.08 0.10

Example 4

In this example, the tribological properties (coefficient of frictionand effect of sliding speed on the dynamic coefficient of friction) weredetermined between the polyoxymethylene composition and varioussurfaces. The polyoxymethylene composition was comprised of 2.8 wt. %UHMW-SI.

In Sample 32, the composition was tested against a polyoxymethylenesurface without UHMW-Si. In Sample 33, the composition was testedagainst a polyoxymethylene composition comprising 2.8 wt. % UHMW-Si. InSample 34, the composition was tested against a polyethyleneterephthalate (PET) surface. In Sample 35, the composition was testedagainst a polypropylene (PP) surface. In Sample 36, the composition wastested against a steel surface. In Comparative Sample 2, apolyoxymethylene sample without UHMW-Si was tested against the samepolyoxymethylene sample.

Stick-slip tests were conducted to determine the dynamic coefficient offriction and the static coefficient of friction. Stick-slip tests wereconducted according to VDA 230-206. A ball-on-plate configuration wasutilized with a load of 12.5 N, sliding speed of 8 mm/s, a test durationof 8 minutes and a test temperature of 23° C.

Comparative Sample Sample Sample Sample Sample Sample 2 32 33 34 35 36Counter POM (w/o POM (w/o POM w/ PET PP Steel Material UHMW-Si) UHMW-Si)2.8 wt. % (HF C9021) (HF C9021) UHMW-Si Dynamic 0.225 0.049 0.035 0.0800.059 0.049 CoF Static 0.239 0.058 0.042 0.092 0.070 0.057 CoF

Testing was also conducted using a load of 30 N and a test duration of45 minutes. Noticeable differences between the results of these testswere not observed.

Example 5

High speed tests were conducted to determine the tribological propertiesagainst an ultrahigh molecular weight polyethylene surface.

A ball-on-prism configuration was utilized with a load of 5 N, slidingspeed of 1000 mm/s, a test duration of 10 minutes. The data wasextrapolated to 60 minutes to determine the long term tribologicalperformance of the materials. The depth of wear in the article was alsodetermined.

Comparative Samples 3-6 include tribologically modified polyoxymethylenewith modifiers other than UHMW-Si. Sample 37 is comprised of apolyoxymethylene composition with 2.8 wt. % UHMW-Si.

Compar- Compar- Compar- Compar- ative ative ative ative Sample Sample 3Sample 4 Sample 5 Sample 6 37 Dynamic CoF 0.58 0.571 0.578 0.616 0.2Depth of wear 4307 7837 5424 3868 490 (μm/hr)

Example 6

High speed, block-on-shaft tests were conducted to determine theabrasive wear against a steel surface. The wear tests were conductedutilizing a steel shaft, a shaft diameter of 65 mm, a roughness of 0.8μm, a load of 3.1 N, a sliding velocity of 136 m/min, a test duration of60 h, and a distance of 490 km.

Comparative Sample 7 utilized polyoxymethylene without tribologicalmodifiers. Comparative Sample 8 was comprised of polyoxymethylene and 18wt. % PTFE. Sample 38 was comprised of polyoxymethylene and 2.8 wt. %UHMW-Si. The surface wear was determined. The wear % reported is basedrelative to Comparative Sample 7.

Comparative Sample 7 Comparative Sample 8 Sample 38 Wear % 100 47 39

Based on the above, the wear percent of Comparative Sample 8 was 47% ofthe wear of Comparative Sample 7. The wear percent of Sample 38 was 39%of the wear of Comparative Sample 7.

Example 7

High speed tests were conducted to determine the tribological propertiesagainst a polyoxymethylene surface that was not tribologically modified.

Stick-slip tests were conducted to determine the dynamic coefficient offriction. Stick-slip tests were conducted according to VDA 230-206. Aball-on-plate configuration was utilized with a load of 5 N, slidingspeed of 1000 mm/s, a test duration of 10 minutes. The data wasextrapolated to 60 minutes to determine the long term tribologicalperformance of the materials. The depth of wear was also determined.

Comparative Sample 9 utilized polyoxymethylene without tribologicalmodifiers tested against the same polyoxymethylene without tribologicalmodifiers. Sample 39 was comprised of a polyoxymethylene and 2.8 wt. %UHMW-Si and was tested against a polyoxymethylene without tribologicalmodifiers.

Comparative Sample 9 Sample 39 Dynamic CoF 0.41 0.12 Depth of wear 80037.8 (μm/hr)

Example 8

In this example, stick slip tests were conducted to determine thecoefficient of friction between a tribologically modifiedpolyoxymethylene and a polypropylene (PP) surface and an unmodifiedpolyoxymethylene (POM) surface.

Stick-slip tests were conducted to determine the dynamic coefficient offriction and the static coefficient of friction. Stick-slip tests wereconducted according to VDA 230-206. A ball-on-plate configuration wasutilized with a load of 12.5 N, sliding speed of 8 mm/s, and a testduration of 8 minutes.

Sample Sample Sample Sample Sample Sample 40 41 42 43 44 45 POMcopolymer (wt. %) (HF C9021) 82 100 94 95 93.25 93 UHMW-Si (wt. %) 0 0 32 2 3 PTFE (wt. %) 18 0 3 3 4.75 4 Stick-slip Dynamic CoF 0.129 0.1220.034 0.047 0.035 0.045 test vs. PP Dynamic CoF 0.078 0.397 0.033 0.0450.035 0.032 vs. POM Modulus (MPa) 2500 2850 2300 2400 2300 2300 Strainat yield (%) 7 9 10 11 11 11 Charpy Notched Impact (kJ/m²) 4 6.5 5 4.64.1 5.3 Sample Sample Sample Sample Sample 46 47 48 49 50 POM copolymer(wt. %) (HF C9021) 92.375 96 91.125 93 92 UHMW-Si (wt. %) 3 1 1 1 2 PTFE(wt. %) 4.625 4 4.875 3 4 Stick-slip Dynamic CoF 0.038 0.062 0.051 0.0660.035 test vs. PP Dynamic CoF 0.032 0.052 0.048 0.043 0.036 vs. POMModulus (MPa) 2300 2400 2400 2400 2300 Strain at yield (%) 11 10.9 11.010.7 10.8 Charpy Notched Impact (kJ/m²) 5 5.4 5.3 5.1 5.3 Sample SampleSample 51 52 53 POM copolymer (wt. %) (HF C9021) 80.9 80.35 79.8 UHMW-Si(wt. %) 2 3 4 PTFE (wt. %) 17.1 16.65 16.2 Stick-slip Dynamic CoF 0.060.048 0.042 test vs. PP Dynamic CoF 0.29 0.026 0.030 vs. POM

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part.

Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention so further described in such appended claims.

The invention claimed is:
 1. A polyoxymethylene polymer composition, thepolyoxymethylene polymer composition comprising; a polyoxymethylenecopolymer having a melt flow index of less than about 50 g/10 min, andfirst tribological modifier comprising an ultra-high molecular weightsilicone having a kinematic viscosity at 40° C. of greater than 100,000mm² s⁻¹, the ultra-high molecular weight silicone being present in thepolymer composition in an amount from about 0.1 to about 10% by weight;a second tribological modifier comprising polytetrafluoroethyleneparticles, ultra-high molecular weight polyethylene, or a silicone oilhaving an average molecular weight of at least 5,000 g/mol and less than75,000 g/mol and having a kinematic viscosity at 40° C. of greater than100 mm² s⁻¹ and less than 15,000 mm² s⁻¹, or mixtures thereof; andreinforcing fibers present in the composition in an amount of at least10% by weight and up to 50% by weight.
 2. The polymer composition ofclaim 1, wherein the second tribological modifier comprises the siliconeoil.
 3. The polyoxymethylene polymer composition as defined in claim 1,wherein the second tribological modifier comprises thepolytetrafluoroethylene particles, the polytetrafluoroethylene particlesbeing present in the polymer composition in an amount from about 0.1 toabout 50% by weight.
 4. The polyoxymethylene polymer composition asdefined in claim 1, wherein the second tribological modifier comprisesthe ultra-high molecular weight polyethylene particles.
 5. The polymercomposition of claim 1, wherein the composition further contains anucleant comprising an oxymethylene terpolymer.
 6. The polymercomposition of claim 5, wherein the nucleant is present in thecomposition in an amount from about 0.01% by weight to about 2% byweight.
 7. The polymer composition of claim 1, wherein thepolyoxymethylene copolymer contains comonomer units in an amount fromabout 0.5 mol % to about 5 mol %.
 8. The polymer composition of claim 1,wherein the polyoxymethylene copolymer has a melt flow index of greaterthan about 5 g/10 min.
 9. The polymer composition of claim 1, whereinthe composition has a modulus of elasticity measured according to ISOTest No. 527 of at least about 2000 MPa.
 10. The polymer composition ofclaim 1, wherein the reinforcing fibers comprise glass fibers.
 11. Thepolymer composition as defined in claim 1, wherein the ultra-highmolecular weight silicone is present in the polymer composition in anamount of from about 1 weight % to about 6 weight %.
 12. The polymercomposition as defined in claim 1, wherein the polyoxymethylene polymeris present in the polymer composition in an amount of at least 70% byweight.
 13. The polymer composition of claim 2, wherein the silicone oilis present in the polymer composition in an amount of from about 0.5% byweight to about 5% by weight.
 14. The polymer composition as defined inclaim 3, wherein the polytetrafluoroethylene particles are present inthe polymer composition in an amount from about 1% to about 10% byweight.
 15. The polymer composition as defined in claim 4, wherein theultra-high molecular weight polyethylene is present in the polymercomposition in an amount of from about 2.5% to about 20% by weight, theultra-high molecular weight polyethylene having a molecular weight ofgreater than about 2.0×10⁶ g/mol.
 16. The polymer composition as definedin claim 1, wherein the polymer composition contains a silicone oil andpolytetrafluoroethylene particles.
 17. A polymer article made from thecomposition of claim 16, the polymer article comprising a gear, a lever,a cam, a roller, a sliding element, a pulley, a latch, a claw, a wiperarm, a conveyor component, a medical inhaler, or a medical injector.